automatic instrumental platform for the measurement...

Turk J Elec Eng & Comp Sci

(2017) 25: 622 – 632

c⃝ TUBITAK

doi:10.3906/elk-1504-183

Turkish Journal of Electrical Engineering & Computer Sciences

http :// journa l s . tub i tak .gov . t r/e lektr ik/

Research Article

Automatic instrumental platform for the measurement of the characteristics of

ferromagnetic materials based on LabVIEW

Monia Ferjani JAAFAR∗

Center for Research in Industrial Engineering (CEREP), Department of Electrical Engineering,The National Higher Engineering School of Tunis, University of Tunis 1, Tunis, Tunisia

Received: 21.04.2015 • Accepted/Published Online: 05.02.2016 • Final Version: 24.01.2017

Abstract: In this paper, we propose a complete automation of the different measurements of ferromagnetic materials

dynamic characterization according to the CEI60404-6 standard. The measurement bench consists of two major parts:

hardware and software. The hardware part comprises a programmable function generator MTX-3240, an electronic card

for the magnetic induction’s waveform control, and an NI PCI-6225 acquisition card. The software part is developed in

the LabVIEW environment to enable the generator and the acquisition card control. It also allows for the processing

of obtained experimental records and the correction of static errors. The identified characteristics are mainly the first

magnetization curve, the static and the differential permeability, the hysteresis loop characteristics, and the core loss

measurements.

Key words: Experimental design, hysteresis loop, magnetic properties, ferromagnetic losses, data acquisition, virtual

instrument, LabVIEW, synchronous detection

1. Introduction

In electrical engineering, it is required to identify the magnetic characteristics of a ferromagnetic material in

order to optimize its design economically and technically. The functioning of the classic electrical devices such

as engines, alternators, or transformers depends on its magnetic circuit characteristics.

With the major progress in digital technology and computer-aided design, several automated magnetic

measuring systems have been developed. Novikov et al. proposed an instrument where the coordinates of the

hysteresis loop points are converted with an analog-digital converter into coupled digital code pairs stored and

processed in a microprocessor [1]. Others developed instruments that are based on the ferrometric approach

consisting of phase-sensitive rectification of signals from the magnetizing and measuring coils of the ferromagnetic

sample. Subsequent averaging is applied to find the instantaneous field intensity and magnetic induction [2].

Krokhin and Sushchev proposed a digital compensating ferrometer that uses a modification of the ferrometric

method [3]. With the progress of virtual instrumentation, Polik and Kuczmann and other research groups

developed different computer-controlled measurement systems applying the National Instruments NI-DAC Data

Acquisition Card and the National Instruments LabVIEW software package [4].

Our efforts in this field concern the improvement of an automatic instrumental platform for the measure-

ment of the characteristics of ferromagnetic materials. We apply an accuracy measurement converter called a

synchronous detector, usually used in the field of instrumentation and digital signal processing. Our method

∗Correspondence: [email protected]

622

JAAFAR/Turk J Elec Eng & Comp Sci

is inspired by the modified ferrometric method and the system is controlled by the NI-DAC and LabVIEW

software.

In this paper, we present the design of a test bench for the automatic magnetic characterization of a

ferromagnetic sample such as a first magnetization curve, static and differential permeability, hysteresis loop,

remanent induction, saturation induction, coercive field, and total losses.

The measurement principle is based on the electromagnetic induction law. The measurement of the

magnetic characteristics is performed on a thin ferromagnetic ring sample in the dynamic regime with a

sinusoidal magnetic induction conforming to the international standard CEI60404-6 [5].

2. Architecture of the measurement system

The essential parameters characterizing a magnetic material, such as coercivity field Hc , magnetization at

remanence point Br , saturation induction Bs , static permeability µstat (Hm), differential permeability µd

(Hm), iron losses PHF (Bm), and features that allow the quantification of losses in the material for a definite

stimulus, are deduced from the hysteresis loop [6]. Figure 1 shows an example of a hysteresis loop with the

essential parameters that can be derived from this characteristic.

Figure 1. Hysteresis loop.

The measurements of the magnetic characteristics of a ferromagnetic sample are based on the identifica-

tion of the hysteresis loop. The identification of magnetic properties of ferromagnetic material requires the use

of a closed magnetic circuit to prevent submitting the sample to an internal demagnetizing field, which might

be the origin of an error source difficult to quantify [7]. For this reason it is recommended to use certain types

of circuits [8]. Moreover, the proposed measures are based on the induction law. The measurement circuit has

two windings, one for excitation and one for measurement. The magnetic flux variation can be obtained by

measuring the induced voltage in the secondary coil [9].

623


However, considering the nonlinearity of electromagnetic phenomena and the need of measurement

standardization of ferromagnetic materials’ characteristics in the dynamic regime, it is necessary to take

measures for a sinusoidal waveform according to the CEI60404 standard. For this purpose, and in order to

have the same operating conditions of an electrical machine and to ensure the reproducibility of measurements,

the control of the waveform requires the presence of feedback. The synoptic plan and the general structure of

the characterization bench of ferromagnetic materials are presented in Figures 2 and 3.

Command and

control unit of the

measurement

system

RS232 PCI

Power

supply

MTX

Acquisition

card

PCI NI6225

Electronic control card of the

stabilization system sample’s

magnetic core

B(t) and H(t)

Figure 2. Synoptic plan of the automatic bench for ferromagnetic characterization of ferromagnetic materials.

Figure 3. Block diagram of automatic bench for ferromagnetic materials characterization with the stabilization system

of the ferromagnetic sample’s magnetic state.

Figure 4 illustrates the constitution of the test bench of ferromagnetic materials characterization. This

system is controlled by a PC via a functional user interface in the LabVIEW environment and it consists of an

624


NI-PCI-6225 acquisition card [10] and a programmable function generator, MTX-3240, with the electronic card

that controls the wave’s form and keeps the sinusoidal form of the magnetic induction B(t).

Figure 4. Test circuit and hardware architecture of the automatic hysteresis measurement system.

The LabVIEW measurement software controls the MTX-3240 generator in terms of amplitude and

frequency via an optic serial port RS232 [11]. This generator can achieve a peak-to-peak amplitude of 20 V

with an automatic calibration and a frequency range from 0.1 Hz to 5.1 MHz. Starting with a sinusoidal signal

set, the servo card keeps the sinusoidal shape of the magnetic induction to ensure measurement standardization.

The voltage representing the magnetic induction VB and the voltage image of the magnetic excitation VH are

acquired thanks to the NI-PCI 6225 acquisition card.

After the design and implementation of an electronic control card corresponding to the sinusoidal magnetic

induction form presented in Figure 5, the output signals obtained are shown in Figure 6. Obtained data will

be processed using the LabVIEW environment and allow calculation of the characteristics of the ferromagnetic

ring-shaped sample mounted on the test bench. The acquisition card is connected to the outputs VB and VH

of the electronic servo card. It has 80 analog input channels, an analog-digital converter with a resolution of 16

bits, and a sampling frequency of 250 kHz.

3. Automation of magnetic measurement in the LabVIEW environment

The control of the signal generator MTX-3240, the acquisition card NI PCI-6225, and the measurement

processing are performed in the LabVIEW environment.

The LabVIEW program for controlling the generator is shown in Figure 7. It consists of Vis for the

initialization of the RS232 communication parameters and configuration settings of the output signal.

625


12

13

14

4

11

U1:D

LM324

R3

10k

RA

130

R5

130

D1

1N4001

D2

1N4001

Q1

MJ11015

Q2

MJ11016

R6

10k

R7

0.5

R8

0.5

R9

10k

3

2

1

4

11

U1:A

LM32

4

R1

10k

RN

0.47

R10

100k

RC

10k

RD

10k

C3

22nF

C4

15p

C2

2200u

5

6

7

4

11 U2:B

LM324

R15

10k

R19

100k

C9

10u

RB

3k

GBF

ERREUR

FEED.B

VB

3

2

1

4

11

U2:A

LM324

RV1

POT 10K

RV2

POT 10K

VB' VH'

C1

47nF

12

13

14

4

11

U2:D

LM324

Q3

2N3703

Q4

2N3704

SW2

SW-SPST-

MOM

+15V

-15V

E42

TORUS

VHN1N2

Ferromagnetic sampleT=-N2/N1

xx

K1= 1/(1-x) with 0>x>1 K2= 1/(1-x) with 0>x>1

Gain of the direct channel :

� = (1+Ra/Rb)

Feedback Gain: � = (1+Rc/Rd)

Summer

Active high-pass filter => Fc=0.159Hz

6DB/octave, gain=1

Power Amplifier

Figure 5. Electronic control card of the magnetic sinusoidal induction form.

Figure 6. Output signals from the control card.

626


Figure 7. Realization of the control diagram of the generator in LabVIEW.

The control program of the acquisition card is shown in Figure 8. It allows acquiring N samples for

a sampling frequency Fe and saving them in a file database. The value of sampling frequency Fe is deduced

from the signal frequency generated by the MTX3240 generator, which provides us with a constant number

of samples N . The magnetic field and magnetic induction are calculated from data obtained by respectively

applying Eqs. (1) and (2).

H(t) =N1i(t)

lmoy(1)

Figure 8. Realization of the control diagram of the acquisition card in LabVIEW.

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B(t) =1

N2S

∫ t

0

VB(t)dt (2)

Here, N1 and N2 are respectively the number of turns of the excitation coil and the sensing coil, lmoy is the

average length of the ring-shaped sample, i(t) is the excitation current measured through the noninductive

low-value resistance RN , S is the section of the sample, and VB (t) is the induced EMF in the sensing coil

terminals. The calculation of the integral in Eq. (2) requires the determination of integration constants that

are not easy to obtain. Thus, the solution is to use the method of synchronous detection as long as VB (t)

satisfies the conditions of periodicity and antisymmetry relations given by Eq. (3).{x(t) = −x

(t+ T

2

)x(t) = x (t+ T )

(3)

T is the signal period.

The instantaneous value of the original signal is known by applying the conversion function of a syn-

chronous detector, which is given by Eq. (4).

X =1

T

∫ ti+T2

ti

x(t)dt = − 2

TX(ti) = −2f X(ti) (4)

x(t) is the derivative of X(t).

According to this principle, the magnetic induction is given by Eq. (5).

1

T

∫ ti+T2

ti

dB(t)

dtdt = − 2

TB(ti) = −2f B(ti) (5)

Here, ti varies from 0 to T .

The program for the conversion function of synchronous detection is shown in Figure 9.

Knowing H(t) and B(t), it is possible to define all the standard magnetic parameters of the ring-shaped

sample:

- The first magnetization curve Bm(Hm), from which static µstat (Hm) and differential permeability µd

(Hm) can be inferred through the implementation of Eqs. (6) and (7):

µstat =Bm

Hm(6)

µd =∆Bm

∆Hm(7)

- The network of hysteresis loops B(H) for different amplitudes of Bm , which then allows to identify the iron

losses PHF (Bm) according to the maximum induction through Eq. (8) [12]:

PHF =f

ρ

∮cycle

H. dB =f

ρ

∮cycle

B. dH [W/Kg] (8)

Here, ρ [Kg/m3 ] is the volumic mass of the ferromagnetic material.

628


Figure 9. Realization of the diagram for calculating the conversion function of synchronous detection in LabVIEW.

4. Results

The test bench for the automatic magnetic characterization has been tested for the ferromagnetic toroidal

sample and for the sinusoidal excitation voltage with a frequency variation from 20 Hz to 400 Hz.

The front panel of the magnetic measurement program is shown in Figure 10. It has 13 display areas for

numerical results, such as Br - remanent induction, Hc - coercive excitation, Bsat - saturation induction, PH

- hysteresis losses, and PHF - core losses.

It has also nine areas of graphic display. In this group, the signal curves VB and VH can be seen at

the top left side. The calculated signals H(t) and B(t) together with the spectral analysis are plotted at the

bottom left. The curves of static and differential relative permeability and the first magnetization curve, as well

as the all-measure hysteresis loop, appear at the bottom right, and 16 control areas can be seen at the top right

side, such as the parameters of the magnetic sample, the magnetic stabilization system, and save location.

Gradually increasing the excitation voltage with a step ∆V , we can evaluate the maximum values of H(t)

and B(t) corresponding to each increase. The first magnetization curve is obtained when these points Bi (Hi)

on the graph are linked. Figure 11 shows the practical result of the first magnetization curve of a toroidal

core sample of ferromagnetic material (Steel E42). Static permeability µstat(Hm) and differential permeability

µd(Hm) are inferred from the first magnetization curve and illustrated by Figure 12.

5. Evaluation of measurement errors

The error in magnetic measurements depends on the sum of measuring channel error and the primary transducer

error. Measuring channel error contains a variety of independent errors such as resolution error, mainly

consisting of the reference voltage and the DAC converter, the sampling error, electronic amplifiers gain errors,

error due to numerical calculation, and other factors.

629


Figure 10. Front panel of LabVIEW measurement software for the automatic characterization bench of ferromagnetic

materials.

–3000 –2000 –1000 0 1000 2000 3000–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

_ !e first magnetization curve_ Hysteresis loops

B [T]

H [A/m] 0 500 1000 1500 2000 2500 3000

0

0.2

0.4

0.6

0.8

1

1.2

1.4

B [T]

µstat 10-3

µd 10-3

_ The first magnetization curve

_ Static relative permeability

_ Differential relative permeability

H [A/m]

B(H)

stat(H)

d(H)

Y axes

Figure 11. The measurement result of the first magneti-

zation curve and hysteresis loops.

Figure 12. The measurement result of the static and

differential permeability. Red graph B (H): The first mag-

netization curve. Blue graph µstat (H): Static relative

permeability. Green graph µd (H): Differential relative

permeability.

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By applying the method of error propagation, the total error is evaluated by this expression:

σtotal =

√∑i

σ2i (9)

Here, σi represents each independent error.

In this automatic magnetic induction and field measurements, the measuring channel error does not

exceed 0.1%.

The primary transducer error is composed of the error due to counting number of turns for the magnetizing

and measuring winding, the error of measuring the cross-sections, and area and length of the mean magnetic

force line of the studied sample, and usually this error is evaluated at 1%.

The total error of magnetic measurements is governed mainly by the primary transducer error.

6. Conclusion

An automatic characterization bench for ferromagnetic materials is successfully achieved. It can automatically

measure various characteristic parameters of a toroidal sample in the dynamic regime according to the latest

CEI60404-6 standard.

This system is simple with autocalibration. It is equipped with a control loop in order to reduce distortion

of the induction caused by the nonlinearity of the phenomenon and with a subprogram self-correcting systematic

errors. The involvement of virtual instrumentation provides the possibility to add new measurement features.

The recorded measurement data can be exported to other environments such as MATLAB. The automatic

characterization bench may be used for identification of hysteresis mathematical models. The operator is thus

relieved from the onerous conventional tasks of measurements and calculations.

Despite having an operational instrument, there is still room for improvements such as:

- The complete automation by automatic search of the bandwidth of ferromagnetic material in terms of

frequency.

- The preliminary identification of saturation parameters.

Acknowledgments

The author thanks the research unit Centre de Recherche en Productique (CEREP) director Professor Anis

Chalbi for financial support, the University of Tunis I, and Ecole Nationale Superieure d’Ingenieurs de Tunis

(ENSIT).

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automatic instrumental platform for the measurement...

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